filter: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/lib/cmtk/bin/filter - from package: cmtk
/usr/bin/filter - from package: filter

You may install the required package with command apt-get install {package}

Error in other languages:
filter: bevel nie gevind nie
filter: comandă negăsită
filter: comando não encontrado
filter: commande introuvable
filter: command not found
filter: käsku ei ole
filter: không tìm thấy lệnh
filter: komanda nerasta
filter: Kommando nicht gefunden.
filter: kommandot finns inte
filter: komut yok
filter: nie znaleziono polecenia
filter: níor aimsíodh an t-ordú
filter: no se encontró la orden
filter: no s'ha trobat l'ordre
filter: opdracht niet gevonden
filter: parancs nem található
filter: perintah tidak ditemukan
filter: príkaz nenájdený
filter: příkaz nenalezen
filter: команда не найдена
filter: командата не е открита
filter: 命令找不到
filter: コマンドが見つかりません